I have an attached 2-car garage that shares 1.5 walls with my house. Due to the large quantity of attached surface area, I think it's worth insulating the garage to keep it a few degrees warmer. The exterior walls are drywalled, but there is no insulation in them, nor in the ceiling (which is also drywall). I have easy access to the ceiling from the attic, and the walls I would either poke holes in and blow in insulation, or tear out to add fiberglass. I don't really want to do that, though.

The garage door (16 foot) is insulated as well.

Worth it? Pros, cons? What about a vapor barrier on the exterior walls and ceiling? Also, the garage attic and house attic share the same airspace...there is no wall in the attic separating them. Should something be done about this?

What exactly do you plan on doing with the garage? Work shop, storage, finished space, etc... will determine if you should only insulate the shared walls with the house and/ or get the attic/ garage attic sealed up.

Well, it was drywalled when we bought it...but whoever drywalled it didn't bother to insulate as well. So, that makes a vapor barrier pretty difficult. Why is it necessary in a garage?

I won't be heating the garage, but it's getting SOME heat from the attached walls, so I thought it would be best to contain that if possible. Plus, heat from two cars could "last" a little longer with some insulation in the winter.

Vapor barrier is used anytime you have moisture that is a problem or can be a problem. Anyplace where cold air and warm meet there will more than likely be condensation.

Personaly I would put a vapor barrier on the walls that are between your garage and living space. For me that would be just more of a preventive measure from mold and rot, but I'm a mold guy so naturally I see mold and water everywhere I look.

The rest of the garage should be fine without any vapor barrier. I personaly would do a vapor barrier just in case I would want to heat it in the future and build a man cave up in there, but if you just want to blow the insulation in there it should be fine as long as you don't have any moisture going on.

You could try knocking out an inspection hole and taking a look at some of the pre exisiting insulation you have. If it is black and crusty looking, you have moisture issues and I would definitly slap up the barrier.

I fully expect the walls common to the house and garage will have insulation and vapour barrier on the house side.

I live in Winnipeg, Manitoba, which isn't all that much colder than New York or Wisconsin. I had a friend that insulated his unheated garage with expanded polystyrene foam (and he didn't use a vapour barrier), and when he'd park his car in the garage and leave it in there overnight, the garage would still be mild temperature in the morning. I don't know what he did in the garage ceiling, I expect he used expanded polystyrene there too. But, he'd park 500 pounds of hot steel in there every evening, and that kept the garage warm all night. But, that's just what he told me. I dunno if he was exagerating or not.

Well, EPS (extruded polystyrene) more or less IS a vapor retarder (perm of ~1 for the 1.5 and 2" stuff, IIRC).

I guess I could use that stuff in the ceiling, since I have access to the attic side of the drywall. I'd have to rip the exterior wall drywall down to install that, though. Would rather just blow it in from a hole in the top of the wall...although I don't know if any fireblocking exists in these particular walls....I wouldn't think so.

Nothing sounds easy at this point. Why couldn't the previous owners have put in insulation before drywalling!!! Grrr..

And yes, as I said before, the walls that are shared with the interior have a vapor barrier (on the interior side). Oh, and the only moisture would be that of the melting snow/ice from 1 or 2 cars (we normally only drive one at a time).

Contact Nu-Wool :: Premium Cellulose Insulation :: Home Page they seem to be able to handle your issue, I have seen them do it before with a high dense blown in, not to mention it is made for use in fire wall construction.
BPI , and building science also recommends them. good luck.

Hi,
My opinion is that if your not going to heat that area for living or working space your wasting your money. The two walls between the house and garage are more sealed or resistant to the outside enviroment than any othe wall of the house just because they have the dead air space of the garage itself. your getting more cold and wind penatration from all other the other exterior walls. If you have a cold air or draft issue, inspect the attic or crawl space for poor insulation job or air penatration.

oh yeah, except that could make it worse using vapor barriers. Look up the DOE and see what they say about vapor barriers......THEY ARE BAD, NOT GOOD, BAD. Anybody still using them knows nothing about const. Vapor retarders, ok, vapor barriers BAD NEWS. I will find the many photos I have of the damage caused by vapor barriers, yours is just a picture of mold, these show you why walls and ceilings HAVE TO DIFFUSE MOISTURE.